Organic fertilizers

In a constantly evolving agricultural context, soil quality and fertility remain central concerns. Humus, which is the sum of dead organic matter in the soil, plays an essential role for all important functions of arable soils. It provides nutrients, improves soil structure, increases its water retention capacity and protects against erosion, while promoting the activity of soil organisms (Niggli et al., 2024; Agroscope.). Soils rich in humus not only guarantee good yields but also better crop resilience to prolonged droughts or intense rainfall (Niggli et al., 2024). This is why organic fertilizers have become a fundamental element of sustainable agricultural practices.

What is an organic fertilizer ?

An organic fertilizer, as its name suggests, is produced from natural organic matter, whether of plant or animal origin(PCC Greenline Blog.). Unlike mineral fertilizers, which are inorganic compounds synthesized from nitrogen, phosphorus, sulfur, magnesium, etc., organic fertilizers are distinguished by their composition based on carbon molecules (PCC Greenline Blog.).

Composition and types of organic fertilizers

Organic fertilizers are a rich source of macro- and microelements essential for the proper development of cultivated plants. They notably contain nitrogen (N), potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), as well as molybdenum (Mo), copper (Cu), manganese (Mn), and boron (B) (PCC Greenline Blog.; Perfarelalbero.it, 2024). However, the quantities of these nutrients are not as precisely defined and adapted to the specific needs of plants as in mineral fertilizer mixes(PCC Greenline Blog.).

Among the common types of organic fertilizers :

Manure (bovine, equine, pig, poultry) and slurry (Foodcom.pl.; PCC Greenline Blog.).
Compost, derived from plant and animal residues, including garden and household waste (Foodcom.p.; PCC Greenline Blog.).
Biohumus, resulting from the decomposition of organic matter by microorganisms and earthworms, especially California worms(Foodcom.p.; PCC Greenline Blog.). It is often used in home cultivation(Foodcom.pl.).
Green manures and cover crops, which are plants grown specifically to be incorporated into the soil to increase its fertility(Beter Bodembeheer.; Inne nawozy organiczne.; Niggli et al., 2024). They contribute to better soil structure and the supply of organic matter (Beter Bodembeheer.).
Crop residues (such as straw or roots), which contribute to the formation of soil organic matter(Inne nawozy organiczne.; Niggli et al., 2024). Harvesting and selling straw for energy purposes is not part of a sustainable approach to soil fertility (Inne nawozy organiczne.).
Other materials like bone meal or meat and bone meal, fish meal, guano, sawdust, garden bark, peat, lignite, and leonardites (Foodcom.p.; PCC Greenline Blog.). Bird droppings (guano) have a very high concentration of easily assimilable nitrogen and phosphates but carry a high risk of over-fertilization (Foodcom.pl.].
By-products of human activity such as municipal and industrial sewage sludge can be valorized, provided that agricultural and ecological requirements and standards for heavy metals and sanitary contamination are respected (Agriculture Durable Genève.). The Pôlebio project in Geneva, for example, aims to transform 48,000 tonnes of organic waste per year into biomethane, 20,000 m³ of biofertilizers, and 12,000 tonnes of compost (Agriculture Durable Genève.). Another project, Pitribon, explores the valorization of urine to produce a complete and odorless fertilizer (Agriculture Durable Genève.).

Advantages and challenges of organic fertilizers

The use of organic fertilizers offers numerous benefits for sustainable agriculture:

Long-term improvement of soil fertility : They increase organic matter content, which improves soil structure, porosity, water and nutrient retention capacity, and aggregate stability (Niggli et al., 2024; Perfarelalbero.it, 2024; Agribios Italiana, 2024). The BioDiVerger soils, for example, showed an increase or stability in their organic matter and a favorable organic matter/clay ratio, indicating good resilience (Guil, 2022).
Stimulation of soil microbial life : They promote the growth and activity of beneficial microorganisms (such as mycorrhizal fungi and nitrogen-fixing bacteria) and earthworms, which are essential for organic matter decomposition and nutrient availability (Agribios Italiana, 2024; Niggli et al., 2024; Perfarelalbero.it, 2024). The BioDiVerger project observed an increase in microbial biomass and earthworm activity, even with low inputs (Guil, 2022).
Gradual release of nutrients : Unlike fast-acting mineral fertilizers that present a high risk of leaching or volatilization losses, nutrients from organic fertilizers are released slowly and over a prolonged period, thus reducing the risks of leaching and groundwater pollution (Agribios Italiana, 2024; Perfarelalbero.it, 2024).
Carbon sequestration : Humus is composed of 40 to 70% carbon and constitutes the largest carbon sink in the soil. Increasing its content helps reduce atmospheric CO2, a major greenhouse gas (Niggli et al., 2024; RTS, 2019b).
Reduction of reliance on synthetic inputs : They offer an alternative to mineral fertilizers, whose production cost is high and supply uncertain (Agriculture Durable Genève.; Lasorella, 2022). Their use contributes to more sustainable agriculture and a circular economy (Agriculture Durable Genève.; Lasorella, 2022).

However, the use of organic fertilizers also presents challenges and considerations

C/N ratio management : A high carbon/nitrogen (C/N) ratio (e.g., lignified straw or lignified cover after winter) can lead to nitrogen immobilization for subsequent crops, as microorganisms use it to decompose organic matter. A low C/N ratio (rich in nitrogen) promotes rapid decomposition and a high nitrogen supply but can increase the risk of leaching losses if the subsequent crop cannot absorb the available quantities (Niggli et al., 2024). A high C/N ratio favors humus formation, while a low ratio increases nitrogen availability (Niggli et al., 2024).
Application conditions : It is important to apply nutrients at the right time, when plants can absorb them, and to avoid bare, waterlogged, very dry soils or during dormancy (Niggli et al., 2024). Large amounts of slurry can harm earthworms, hence the recommendation not to exceed 25 m³ per hectare per application, or to dilute it (Niggli et al., 2024).
Tillage : Excessive or intensive tillage can degrade humus and lead to organic matter losses (Niggli et al., 2024). Reduced plowing promotes the accumulation of organic matter in the topsoil and soil life (Niggli et al., 2024; Guil, 2022).
Product quality and contaminants : The quality of organic fertilizers varies, especially regarding water content. It is crucial to choose certified products from reliable companies (Foodcom.pl.; Niggli et al., 2024). The import of pathogenic organisms or problematic weeds can be avoided by sourcing from reliable sources (Niggli et al., 2024). Organic fertilizers, especially those from industrial digestates or green waste compost, may contain foreign substances like plastic, which can accumulate in soils with regular use (Niggli et al., 2024).
Regulation : The new European Regulation (EU) 2019/1009, in force since July 16, 2022, aims to harmonize the placing on the market of EU fertilizers, including organic fertilizers and biostimulants, thus encouraging their use for more sustainable agriculture (Lasorella, 2022; EU Fertilizers.). This regulation establishes strict quality and safety standards for products, including limits for specific contaminants and organic pathogens (EU Fertilizers.). It also requires technical documentation and conformity assessments (EU Fertilizers.). Specific analytical methods exist to determine the quality of organic fertilizers, including organic carbon content, humification degree, blood presence, fungal biodiversity, or biodegradability (Ministero delle Politiche Agricole, Alimentari e Forestali, 2000).

In summary, organic fertilization is an essential agronomic strategy that feeds the soil rather than directly the plant (Perfarelalbero.it, 2024). By adopting appropriate practices, it contributes to building a resilient agricultural system capable of adapting to climatic challenges while improving crop productivity and quality (Niggli et al., 2024; Guil, 2022). Studies have shown that the use of compost and granular organic fertilizers can significantly increase the yields of crops such as cucumber and broccoli in organic farming (Kowalski & Matysiak, 2021; Kowalski & Matysiak, 2022a).

References

Agribios Italiana. (2024, 2 septembre). Scegliere il concime corretto: una guida.

Agriculture Durable Genève. (s.d.). Fertilisation des sols.

Agroscope. (s.d.). Agroscope Humusbilanz. https://www.humusbilanz.ch/

Beter Bodembeheer. (2025). De juiste groenbemesterkeuze is essentieel voor goed resultaat.

Dussán López, P. (2023). Land health monitoring framework. Towards a tool for assessing functional and habitat diversity in agroecosystems. IUCN Common Ground in Agriculture Series No. 1. IUCN. https://doi.org/10.2305/LCRH6058

Foodcom.pl. (2024, 11 janvier). Czym są nawozy organiczne? Rodzaje i ich zastosowanie.

Guil, S. (2022, 30 novembre). Rapport sur la qualité des sols du BioDiVerger. Institut de Recherche de l'Agriculture Biologique FiBL.

Inne nawozy organiczne. (s.d.). Inne nawozy organiczne. Polskie Stowarzyszenie Zrównoważonego Rolnictwa i Żywności.

Keith, D. A., Ferrer-Paris, J. R., Nicholson, E., & Kingsford, R. T. (Eds.). (2020). The IUCN global ecosystem typology. IUCN.

Kowalski, A., & Matysiak, B. (2021). Ocena wpływu nawozów organicznych oraz preparatów mikrobiologicznych na wzrost i plonowanie ogórka i brokułu w uprawie ekologicznej. Instytut Ogrodnictwa – PIB, Skierniewice.

Kowalski, A., & Matysiak, B. (2022). Ocena wpływu nawozów organicznych oraz preparatów mikrobiologicznych na wzrost i plonowanie ogórka, brokułu i marchwi w uprawie ekologicznej. Instytut Ogrodnictwa – PIB, Skierniewice.

Lasorella, V. (2022, 21 juillet). Finalmente in vigore il Nuovo Regolamento dei Fertilizzanti: domande e risposte. AgroNotizie.

Ministero delle Politiche Agricole, Alimentari e Forestali. (2001). Metodi applicabili ai concimi organici, organo-minerali, ammendanti e correttivi. Gazzetta Ufficiale della Repubblica Italiana, 21, Supplemento Ordinario n. 6.

Niggli, J., Böhler, D., & Schmid, T. (2024). Gestion de l’humus – Humification: maintenir et améliorer la fertilité du sol. Institut de recherche de l’agriculture biologique FiBL. https://orgprints.org/id/eprint/53281/

PCC Greenline Blog. (s.d.). Czym są nawozy organiczne i co warto o nich wiedzieć?.

Perfarelalbero.it. (2024, 26 octobre). Guida completa alla concimazione: quando, perché e quale prodotto scegliere.

Phillips, H.R., Guerra, C.A., Bartz, M.L., Briones, M.J., Brown, G., Crowther, T.W., Ferlian, O., Gongalsky, K.B., Van Den Hoogen, J., & Krebs, J. (2019). Global distribution of earthworm diversity. Science, 366(6464), 480–485. https://doi.org/10.1126/science.aax4851

Regolamento europeo dei fertilizzanti un nuovo inizio per il settore. (2022). [Presentation slides].

RTS. (2019, 12 avril). SOLS 5/5 - Capturer le CO2 [Audio podcast]. Vacarme.

Van Den Hoogen, J., Geisen, S., Routh, D., Ferris, H., Traunspurger, W., Wardle, D.A., De Goede, R.G., Adams, B.J., Ahmad, W., & Andriuzzi, W.S. (2019). Soil nematode abundance and functional group composition at a global scale. Nature, 572(7768), 194–198. https://doi.org/10.1038/s41586-019-1418-6

This page has been written for the NBSOIL project, with the financial help of the European Union, with the help of the Centre National d'Agroécologie, of Ver de Terre Production and of Neayi